1.
Introduction
The authors are right to emphasize that Vendozoa probably include both filter feeders
on plankton and those deriving nutrition directly from the underlying substratum on
which they lie or move and to argue that hard and soft surfaces would have provided
habitats offering partially contrasting selective forces favouring distinct body forms.
However, my paper already argued that some Vendozoa were bifacial filter-feeding fronds
and others likely to have been horizontal dwellers on soft surfaces that may have
fed phagocytically on substrate microrganisms by ventral non-choanocyte cells and
dorsally on plankton by choanocytes. A better interpretation of Fractofusus than that
of Dufour & McIlroy [1] might be that it was just such a dorsal collar-cell and ventral
substrate feeder; if so it was a presponge, not a pre-placozoan. If Ediacaran organisms
of that dual feeding mode existed, the dichotomy between plankton feeders and substrate
feeders was less sharp than they imply.
Their comment raises seven issues: (i) conceptually, how does the ‘pre-placozoan grade
of organization’ really differ from a presponge. (ii) Can we reliably infer from fossils
the actual feeding mode and different cell types of Vendozoa? (iii) What are the phylogenetic
relationships between Vendozoa, sponges and Placozoa; and the inferred phenotypes
prior to each branch point? (iv) What is the relative timing of vendozoan, sponge
and bilaterian origins? (v) Does substrate feeding offer a transition to the first
animal as plausible as the choanoflagellate to presponge path? (vi) What selective
force was crucial for the origin of the nervous system? (vii) What is required for
good explanations of major evolutionary transitions?
2.
‘Pre-placozoan grade’ is conceptually confused
McIlroy and Dufour introduced the term ‘pre-placozoan grade’ to apply to the earliest
rangeomorph fossils [2], but did not define or clearly explain it. They now say pre-placozoans
‘differ from presponges by their lack of basal pinacocytes and suspension feeding
capabilities, and by a feeding mode that relies on establishing symbiosis with, or
directly phagocytosing chemoautotrophic bacteria’ [1]. The hypothetical nature of
bacterial prey is not helpful for defining a grade of organization—a morphological
concept of organismal substructure. Thus, using it implies that an organism with identical
structure (thus organizational grade) feeding not on chemotrophic but on heterotrophic
or photosynthetic bacteria or on eukaryotes or osmostrophically would not be of ‘pre-placozoan
grade’, which is unreasonable. Excluding prey type from this definition leaves only
lack of basal pinacocytes, which does not adequately define a grade of animal organization.
The authors also confusingly note that compared with presponges ‘The pre-placozoan
model presents a similar body plan (simple epithelium surrounding mesohyl), but with
only choanocyte-like cells, involved in feeding and water circulation’ [1]. A ‘similar
body plan’ is effectively the same as having a similar grade of organization, yet
if they had only ‘choanocyte-like cells' their grade of organization would be that
of a multicellular choanoflagellate (if ‘choanocyte-like’ means having a microvillar
collar), not an animal, and thus a simpler grade than presponges. If they had only
cells with collars as they imply, their dorsal cells should have been able to feed
on plankton exactly like a presponge, so asserting that they did not feed on plankton
but only from their ventral surface seems contradictory, and would have lowered their
feeding effectiveness and thus have been selected against. Furthermore, it is unlikely
that their ventral surface would retain collars, in which case saying they had only
choanocyte-like cells seems wrong. Conversely, if they are not postulated to have
had collars, it was wrong to call them ‘choanocyte-like’. Thus, properties suggested
for ‘pre-placozoa’ are partially contradictory and/or too loosely specified.
Earlier, the authors said the vendozoan Dickinsonia ‘was of placozoan grade’ with
‘mucociliary sole’ [2], but neither then nor now specify how a ‘pre-placozoan grade’
differs from a ‘placozoan grade’. As I use ‘presponge’ [3], it embraces a spectrum
of increasingly complex grades of organization with two to many different cell types,
ranging from the simplest with two layers (true diploblasts; epithelium only) to others
with mesenchyme also and thus three tissue layers (simple triploblasts). Thus, saying
it ‘consisted of two cell types' [1] oversimplifies and misrepresents my paper. I
would regard the most complex presponges as having a similar grade of organization
to Placozoa (albeit one of their cell types being choanocytes).
3.
Are any Vendozoa ‘pre-placozoa’?
One cannot see cells or cell types in fossil Vendozoa, so we are unsure whether they
had ciliated cells or choanocytes, but can make an informed guess. Their overall size
and complexity is such that it is unlikely that they were not triploblastic with the
connective tissue secreted by mesenchyme cells sandwiched between two epithelia. Placozoa
are also, in this sense, triploblasts, and the blanket use of diploblast for any pre-bilaterian
animal [2] is thoroughly confusing and ought to cease. If they have neither gut nor
aquiferous system, they can reasonably be regarded as of comparable organizational
grade to Placozoa. But, it is not reasonable to regard any of them large enough to
be found as fossils as of ‘pre-placozoan’ grade if that means organizationally simpler
than Placozoa. In my view, all must have had substantially more complex connective
tissue organization than Trichoplax, so the authors' idea that they were organizationally
simpler than Placozoa is implausible; that Vendozoa might have had only choanocyte-like
cells is incredible. If Fractofusus had the tissue structure postulated earlier, it
would have been a complex triploblast not a diploblast as stated [2]. Calling it a
pre-placozoan (without defining that term at all) allowed the reader to think they
supposed it to be an evolutionary precursor of Placozoa. Their present attempt to
give ‘pre-placozoan’, a meaning makes it evident that Fractofusus was not a pre-placozoan,
so their title [2] was doubly misleading and conceptually confused.
4.
Phylogenetic issues
Dufour & McIlroy's hypothetical tree [1] assumes that pre-placozoa gave rise to Placozoa,
coelenterates and Bilateria, but not sponges; like the text, it appears to assume
that the last common ancestor of pre-placozoa and sponges (i.e. all animals) was a
multicell with only one somatic cell type (presumably choanocyte-like) that secreted
extracellular material, yet had already evolved oogamy. It would have been effectively
a multicellular choanoflagellate with oogamy. They do not suggest how this hypothetical
animal ancestor fed or what selective force might have made it evolve oogamy (extremely
rare in protists; unknown in any members of kingdom Protozoa) or multicellularity
and is thus explanatorily empty with respect to the origin of animals. If it fed on
plankton using a collar, as it must have done if choanoflagellate and sponge collar
cells are homologues, it is effectively a presponge in structure and mode of feeding,
so the last precursor of all animals would have been functionally a presponge, making
it incorrect and misleading to say that ‘pre-placozoa’ offer an alternative route
to the origin of animals. At best, their pre-placozoan would be relevant to the origin
of the sister clade to sponges, but no good case is made for that.
The scheme also does not specify how the pre-placozoan, which supposedly evolved later
than this original collar-cell feeding animal, differs from it in structure or feeding
mode, and is thus equally explanatorily empty with respect to the origin of this subclade.
It also has nothing to say about the origin of Placozoa, coelenterates or bilateria.
Worse still, making the animal ancestor effectively a very simple organism with only
one somatic cell type totally fails to account for the sharing by sponges and their
sister clade of a common system of pattern formation, e.g. the Wnt antero-posterior
gradient system, Notch signalling, homeobox and other spatially controlled switch
genes, or the shared PIWI germline maintenance system [3]. It is implausible to argue
either that such complex systems could have evolved at a choanoflagellate grade of
organization with only one somatic cell type and no germline soma distinction or that
they evolved separately in sponges and other animals; therefore, their fig. 1 idea
of such a simple last common ancestor is almost certainly wrong. Furthermore, sponges
and cnidaria/bilateria share an internal body cavity and mouth/osculum and an adult
grade of organization immensely more complex than the pre-placozoan; these complexities
are arguably homologous morphologically and in their pattern-forming gradient system
and germ–soma distinction. Moreover, sponges and cnidaria share a common life history
with a well-differentiated ciliated larva that uses aboral/aboscular secretory cells
that mediate larval settling in similar ways and in sponges express numerous post-synaptic
protein homologues. Despite these fundamental homologies in body plan, pattern formation
and morphogenesis of both adults and larvae of sponges and cnidaria, as well as in
the likely transition mechanisms between them, Dufour & McIlroy ([1], fig. 1) assume
that these complexities evolved independently twice in animal evolution. That is contrary
to the comparative evidence and evolutionarily incredible.
Contrary to their assertion that pre-placozoa offer an alternative route to sponges
for the origin of animals, their fig. 1 shows it as an additional route, making two
almost independent origins of the basic animal body plan and biphasic life cycle.
This two-origin scenario doubles the complexity of animal origins. No intermediates
or causes are given of how their proposed second route could have converted a one-somatic-cell
ancestor directly into a cnidarian or bilaterian as fig. 1 implies, so it is a non-starter
as an ‘explanation’. Merely calling their hypothetical common ancestor a ‘pre-placozoan’
without specifying even one way it differs from its ancestor shared with sponges is
not an explanation. It is also misleading to call this entity a pre-placozoan which
implies that it was a direct ancestor of placozoa, when it is likely that Placozoa
evolved by simplification from a much more complex ancestor with numerous cell types.
It was no more sensible to call it a pre-placozoan than a pre-coelenterate or pre-bilaterian
if the authors' tree is correct.
Their fig. 1 arbitrarily put ‘basal pinacocytes’ at the base of the sponge-only lineage;
they did not explain why they could not have been put below the basal fork and why
their pre-placozoan could not have evolved from a presponge with two somatic cell
types. The authors acknowledge that colonization of the surface of soft marine sediments
is challenging, but do not explain how a multicellular choanoflagellate could have
overcome those challenges or tell us anything about intermediates.
Oddly given their palaeontological expertize, they did not place Vendozoa on their
fig. 1 which makes it hard to fathom what they really think is the relevance of Vendozoa
to either the origin of animals or the primary bifurcations of extant groups, which
limits one's ability to sort the wheat from the chaff in their comment, but I assume
they would have put them as sister to the placozoa/cnidaria/bilateria clade had they
not preferred to hide their view by omitting them. My own interpretation was that
Vendozoa are derivatives of a complex triploblastic presponge grade of organization
(with connective tissue as complex as Cnidaria or sponges), but without aquiferous
or nervous system or gut or nematocysts, which had acquired triploblastic tissue organization
by the choanoflagellate to advanced presponge route, but diversified to fill a variety
of nutritional niches before a gut or nervous system evolved. None of their references
to the fossil record contradicts that view. Non-descript Thectardis might, as they
suggest, be a sponge, but it is hard to say what it is—if it is an early animal, its
size suggests triploblasty not a pre-placozoan grade. It is plausible that Blackbrookia
is a sponge before mineralized spicules evolved, but one cannot be sure.
5.
New palaeontological evidence on relative timing of eukaryotic kingdom origins
A recent study (taking more care to exclude modern contaminants than before) concluded
that 24-isopropylcholestane (ipc) sometimes supposed to be a specific marker for demosponges
does not extend backwards prior to approximately 650 Ma in the extensive interglacial
between the major Sturtian and shorter Marinoan Snowball-Earth episodes in the Cryogenian
period [4]. If ipc really were a specific demosponge marker, that would mean that
sponges originated a few tens of millions of years before Vendozoa and ought to be
widespread in the Ediacaran fossil record, yet possible sponges are sparse (likely
stem not crown sponges) and convincing demosponges or hexacts absent. However, as
Antcliffe emphasized [5], ipc is also made in large amounts by pelagophyte algal chromists,
a deep branching ochrophyte lineage, so its sudden late-Cryogenian rise might be attributable
to the origin of planktonic ochrophyte algae, not sponges. That is quite plausible
as ipc first appeared abundantly at precisely the same time as stigmastane, held to
mark the origin of Viridiplantae, which are sisters of red algae whose symbiogenetic
enslavement originated kingdom Chromista (oddly not mentioned by Brocks et al. [4]),
the sister of kingdom Plantae, which most likely happened rapidly after the origin
of red algae—necessarily close to the origin of Viridiplantae [6,7]. Thus, evidence
from ipc is asymmetric: its absence before 650 Ma makes it unlikely that sponges or
other animals evolved before then, but its appearance then does not require sponges
to be that old or older than or as old as Vendozoa. Probably, the simplest presponges
were slightly older than Vendozoa.
This conclusion is unaffected by an unconvincing claim that pelagophytes evolved the
ability to make ipc 100 Myr after demosponges [8]. That was based on assuming that
this now unquestioned independent ipc origin required independent gene duplications
for carbon-24/28 sterol methyltransferase (SMT), for which there is no direct evidence
as the exact enzymes used and their degree of multifunctionality (widespread in steroid
synthesis) are unknown, combined with a flawed attempt to date sponge and chromist
SMT duplications [8]. Motivating logic was reasonable, but the conclusion was entirely
invalid as methods were extremely biased: their protein-sequence tree (fig. S3A) grouped
the third pelagophyte paralogue with 79% support not with other heterokont (=stramenopile,
a regrettable junior synonym) sequences, but with that of the alveolate Perkinsus,
showing it is not a recent intra-heterokont duplication, but probably occurred before
the last common ancestor of Halvaria (heterokonts plus alveolates). Yet, their fossil-calibrated
molecular ‘clock’ analysis did not use this protein tree, but a taxonomically much
sparser one from which Perkinsus and all other non-heterokont chromist sequences (originally
too sparsely sampled) were omitted and used less reliable nucleotide not amino acid
sequences. On that unwisely culled tree, the pelagophyte third paralogue had no close
relative, so switched its position from being with heterokont clade 2 to beside heterokont
clade 1 (fig. S3B). Worse still, the authors did not even use that tree for their
analysis, but first put it through a programme (NOTUNG) that changed its branching
order (!) to minimize duplications that moved the pelagophyte paralogue 3 into heteterokont
paralogue 1 as sister to pelagophyte paralogue 2, making it falsely appear in fig.
3 to result from a relatively recent duplication, the exact opposite of what the more
reliable, unmanipulated protein tree (fig. S3A) shows. Even if the input had not been
thus topologically seriously distorted, using any single-gene paralogue tree to date
events would likely have been highly unreliable (the grossly inflated earlier dates
on their fig. 3 are not credible, such backward extrapolation being far too model-dependent
to be useful); by comparison with multigene trees, their heterokont branching order
was completely wrong for both paralogues.
If the third halvarian paralogue really does add the third methyl to make pelagophyte
ipc [8] (questionable), its origin would be better dated by mapping it onto a multiprotein
tree (e.g. [7], fig. 2); from that tree for 187 proteins and 171 eukaryotes (opisthokonts
and chromists much more richly sampled than in [8]), Halvaria appear substantially
older than opisthokonts, which (if the authors' assumption of the significance of
that duplication is correct) makes it likely that the oldest ipc signal was not from
sponges, but from halvarian algae and demosponges evolved significantly after 650
Ma. Thus, new sterane data [4] are compatible with my suggestion that presponges and
thus animals originated at the end of the Cryogenian [3], likely almost immediately
after Marinoan melting removed the last inhibitory ice-house conditions that previously
likely restrained diversification of pre-existing protozoan phylum Choanozoa; I suggest
that choanoflagellates (not the oldest Choanozoa) arose then, before which animal
origin was evolutionarily impossible, because the transition was too hard selectively
through any route other than choanoflagellates to sponges.
It may also have been impossible before then for environmental reasons, because the
latest biogeochemical theory of atmospheric oxygen levels argues for three separate
long-enduring metastable global steady states differing by over two orders of magnitude
(Archaean low pO2; most Proterozoic intermediate pO2; Ediacaran/Phanerozoic high pO2)
separated by two sudden Snowball-Earth destabilizing eras that rapidly increased oxygen
levels; and that Cryogenian snowball perturbations switched the medium level to the
present highest level [9]. Though that inferred sudden rise in oxygen presumably provided
a permissive environment for animal evolution for the first time [9], I argue that
the concomitant origin of choanoflagellates and their unique filter-feeding was the
key positive stimulus by providing the only suitable cellular precursors.
This scenario is not affected by the fact that a novel sterane, tentatively identified
as 26-methylcholestane (cryostane) and suggested as a demosponge marker [10], is restricted
to the later part of the immediately preCryogenian Tonian period. Demosponges being
the only organisms known to methylate sterols in the 26-position is not convincing
evidence that they made that cryostane, especially as sponges thus methylate ingested
ergostane and stigmastane (absent before 650 Ma) not cholestane, and thus do not synthesize
the right steroid precursor for making cryostane. If despite those objections cryostane
were from demosponges, why should it have disappeared from the fossil record approximately
720 Ma? As no analyses exist for sterols in most heterotrophic protist lineages, some
might make cryostane. More likely, cryostane was made by a now extinct protozoan that
flourished only approximately 770–720 Ma, e.g. the marine vase-shaped probable testate
amoebae like Melanocyrillium that I argued were probably an extinct early group of
amoebae [11], not arcellinids (as palaeontologists assumed) which are exclusively
freshwater. These flask-shaped tests closely fit cryostane's temporal duration [12].
If the sudden jump in sterane/hopane ratios at 650 Ma and simultaneous onset of the
ipc record in the Sturtian/Marinoan interglacial represents the origin of Plantae
[4] and Chromista, then the simultaneous origin of 24-n-propylcholestane, a putative
marker for the heterotrophic chromist infrakingdom Rhizaria [4], is precisely what
is expected from the chromist diversification pattern on multigene trees [7]. That
makes it likely that neokaryotes (the clade comprising animals, fungi, protozoan subkingdom
Neozoa, Plantae and Chromista [6]) substantially diversified in a neokaryote explosion
approximately 650 Ma. The absence of fungi and choanoflagellates before then, both
able to make ergosterol, would account for ergostane rarity. Before neokaryotes originated,
eukaryotes probably comprised only heterotrophs from protozoan subkingdom Eozoa (phyla
Euglenozoa, Percolozoa and Eolouka [6]), of which kinetoplastid Euglenozoa at least
make ergosterol and could have contributed to the isolated late Tonian ergostane occurrences
(though if contrary to my assumption, the root of the eukaryote tree is not within
Eozoa, but between Eozoa and neokaryotes (as one sequence tree suggested), then the
pre-650 Ma sterane record dominated by cholestanes might have come largely or entirely
from stem eukaryotes not Eozoa). If neokaryotes originated just after or just before
the Sturtian glaciation, the somewhat earlier vase-shaped fossils would have represented
an extinct eozoan group not early Amoebozoa.
The now decontaminated sterane fossil record [4], including the conspicuous absence
of steranes (thus eukaryotes) from an approximately 820 Ma hypersaline habitat where
the oldest convincing isoprenoid evidence for archaebacteria (in my view, sisters
not ancestors or eukaryotes) was found [13], fits my longstanding conclusion that
crown eukaryotes and archaebacteria are both substantially younger than many palaeontologists
think and that all fossils older than approximately 810 Myr identified as crown eukaryotes,
e.g. Bangiomorpha claimed to be a red alga, were misidentified [12,14]. However, sterane
data before 820 Ma are still absent, but essential to test this more rigorously (and
likely disprove the entirely unfounded dogma of archaebacterial antiquity). Even the
present evidence shows that eukaryotes had only a minor ecological role before 650
Ma and a much narrower range of steranes than modern eukaryotes, all simpler C26,
C27 cholestanes except for cryostane unknown from crown eukaryotes, as one might expect
if they were largely stem eukaryotes.
As Mills & Canfield plausibly explain [15], the origin of efficient planktonic filter-feeding
by sponges (and I now suggest by the more complex presponge frondose Vendozoa) probably
decreased the availability of their picoplankton prey and simultaneously seeded shallow
sediments with novel detrital microparticles that would have increased the food supply
and selective advantage for benthic microbes and sediment-feeding animals, an ecosystem
engineering event amplified by the almost immediate consequential origin of coelenterates
(enabled by the complex body plan and pattern formation of their putative stem sponge
ancestors), thereby magnifying the adaptive zone available for through-gut vermiform
bilateria, which likely evolved soon afterwards and diversified rapidly in the late
Ediacaran, generating the early Cambrian fossil explosion—a virtually inevitable tertiary
consequence of triploblasty, neurogenesis and the gut, not the direct result of multicellularity
per se. Thus, for these ecological reasons, as well as on the tissue evolution principles
I emphasized [3], it makes most sense for benthic feeders to have followed sponge-like
planktonic filter feeders.
6.
Huge feeding capacity loss in the pre-placozoan model
Contrary to the unjustified assertion that the pre-placozoan ‘actually shows no reduction
in feeding capacity’ at the unicellular to multicellular transition [1], it implies
a halving of feeding capacity, as they assume that its dorsal epithelial cells do
not engage in feeding by collar cells and only the ventral epithelium feeds on substrate
microbes. Compared with a multicellular choanoflagellate ancestor where all cells
ingest, it would be at a twofold selective disadvantage and thus could not have evolved
without a compensating selective advantage. As the authors failed to recognize their
model's inherent twofold disadvantage, they did not even try to propose a compensating
advantage or indeed specify any selective advantage of muticellularity. As I stressed,
though evolving multicellularity is mechanistically easy (cell surface glue), it is
normally selected against, which is why there are hundreds of thousands of unicell
species, and can only evolve with a substantial selective advantage [3]; their scenario
beautifully exemplifies the scores of disparate ideas put forward for animal origins
that avoid specifying the selective advantage that favoured the assumed intermediate
state, which usually would be disadvantageous making them causally non-explanations
of the problem. The choanoflagellate to sponge pathway remains the only one that allows
a transition to animal multicellularity with several somatic cell types without gross
selective disadvantage.
Dufour & McIlroy [2] did not originally argue that a Fractofusus-like animal was the
first animal. Their new idea that a pre-placozoan could have originated directly from
a multicellular choanoflagellate and gave rise directly to cnidaria is evolutionarily
unsatisfactory through omitting detailed intermediate stages and selective advantage
arguments. The proposed feeding modes (intracellular symbiogenesis of chemosynthetic
bacteria or bacterial phagotrophy on soft sediments) are extensively exploited by
hordes of different protists, but no multicells lie or move on surfaces feeding in
the proposed manner. In my view, it is more efficient to do both as a unicell and
selection would act predominantly against evolving such multicelluarity and thus prevent,
not favour, it; they mention multicellularity increasing surface area, but it actually
reduces the surface area to volume ratio compared with unicellularity. No argument
was given why multicellularity mutants relying nutritionally on ventral epithelial
feeding could survive by abandoning dorsal collar-based filter-feeding in competition
with competitors that retained it in addition to ventral feeding, as their new model
implicitly assumes.
However, after the difficult transition to triploblasty was already made by the presponge
route on hard substrates, the situation would be entirely different. Then, flat quilted
Fractofusus-like organisms could have colonized previously unexploited soft muddy
substrates by filter-feeding triploblastic precursors detaching from solid surfaces,
flopping down onto the mud and focusing ventrally on epithelial non-filter-feeding
phagotrophy (like Trichoplax) as I already proposed [3] and maybe also getting nutrients
from intracellular symbionts for the reasons emphasized by Dufour & McIlroy [2]. That
would exploit a new adaptive zone through a highly developed precursor with far greater
chance of success than a simple pre-placozoan; this habitat change would have evaded
competition from their vertical bifacially choanocyte-feeding forbears, so they could
have evolved even if losing collars ventrally initially lowered feeding efficiency.
But, it would be better for such benthic feeders to have retained dorsal collar-based
feeding to minimze the cost in lost food, so I do not understand why Dufour and McIlroy
insist on total loss of choanocytes—to make them appear not to have been of advanced
presponge grade, as I considered them?
7.
Neurogenesis and selective forces
Seemingly to undermine my explanation of neurogenesis, McIlroy and Dufour asserted
that ‘the mode of life of the presponges and sponges as suspensivores/osmotrophs on
hard surfaces is simple and has few selection pressures requiring the evolution of
a nervous system’ [1], which embodies several evolutionary fallacies. First is the
centuries-old myth of sponge simplicity compared with other animals, which valiant
efforts of sponge specialists for decades (e.g. [16–18]) have evidently not yet expunged
from the secondary zoological literature, as it should be. Second is the quasi-Lamarckian
misconception that selection ‘requires’ evolution of complex characters. Mutations
cause change; selection is simply the metaphor for the inevitable dying out of those
reducing reproductive success and multiplication of those whose effects on development
increase organismal reproductive success, not a separate force of nature [19]. It
does not matter that there may be only ‘few’ selective forces favouring something.
Only one reason is needed to favour a novelty decisively when it arises. The authors
ignore my suggestion that the decisive step in nervous system origins was modifying
the settling mechanism of sponge larvae, converting their flask cells into nematocytes,
initially for improving a pre-existing feature of the complex life cycle: better coordination
between larval sensors detecting a good place to settle and secretion of extracellular
glue to fix the adult [3]. Sensory control over settling, when sensors and effectors
are separate cells, is a shared feature of the almost equally complex biphasic life
cycle of sponges and Cnidaria, which would not have been shared by the purely benthic
‘pre-placozoan’. It was probably later recruitment of incipient nervous control of
proto-nematocytes for a novel feeding mode on larger prey that perfected the nervous
system [3].
Even if later in history the same set of early mutations occurred in other sponges,
they would not have evolved nematocyst-mediated carnivory and a true nervous system,
because crude beginning forms like those arguably successful in the absence of Cnidaria
could not have competed with fully evolved Cnidaria, so selection would have prevented
repetition of earlier events (a major principle in evolution: the chief spoils go
to whoever first collars the market and excludes start-up competitors; the harder
it is to duplicate the key innovation or steal it by symbiogenesis as first plants
and then chromists did for chloroplasts [6] the more likely is enduring monopoly,
best exemplified by the single origin of eukaryotes, the hardest step in all evolution
[20]). Therefore, when cladorhizid demosponges independently evolved carnivory, they
evolved neither nematocysts nor a true nervous system, but some were so radically
changed as to lose the aquiferous system [21], yet unlike stem cnidarians did not
originate a suite of new phyla as all major animal adaptive zones were already filled.
That proves that sponges have the potential to become carnivores and radically change
their body plan, but consequences of such changes inevitably differ depending on what
other organisms are present. (Incidentally, glass sponges evolved action potentials
independently of Cnidaria and some glass sponges colonized soft surfaces secondarily.)
The authors neither argue against my neurogenesis idea nor suggest why or how a far
simpler pre-placozoan instead might evolve a nervous system or offer any reason why
it should have had a complete post-synaptic scaffold, as was present in the last common
ancestor of sponges and other animals [22].
8.
Requirements for good explanations of major evolutionary transitions
I have explained why the comment altogether fails to explain the origin of animals
or any major extant subgroups or to make a case for the feeding mode postulated for
Fractofusus being that of the last common ancestor of the placozoa/Cnidaria/Bilateria
clade; and why their assumed diphyletic origin of animals is almost certainly wrong
and the pre-placozoan idea imprecise and confused. Yet, on the positive side, their
idea of how Fractofusus fed might be partially correct, and I agree that both substrate
phagotrophy and endosymbiosis of chemosynthetic bacteria are possible nutritional
modes for some Vendozoa. But, one must not confuse nutritional mode with body plan
or organizational grade or confuse the diversification of Vendozoa with either the
origin of animals or the causes of the Cambrian explosion: three distinct questions.
It is also essential to consider selective advantages of proposed major transitions;
to specify assumed intermediate stages in more explicit detail than they did; to recognize
molecular and developmental evidence for animal organizational unity; and to have
more respect for Occam's razor, which can readily cut away most of what has been written
on animal origins. Evolutionary hypotheses should be explicit and detailed to facilitate
reasoned criticism, refutation and improvement; not non-committal and vague to evade
referee objections which can make them untestable and scientifically useless.